The ultimate goal of this proposal is to develop a biologically based model of the dentate gyrus, a subsystem of the hippocampal formation. Activity patterns propagate through the five hippocampal subsystems in a largely sequential and unidirectional fashion and undergo significant non linear transformations in each. These transformations appear to be necessary for learning and memory functions, as damage to the hippocampal formation in rats, monkeys, and humans produces severe impairments on memory-related tasks. One of the long-term goals of this lab is to quantify the ways in which activity patterns are transformed by the hippocampal formation. We are attempting to quantify these transformations using an approach based on systems and control theories called nonlinear systems analysis. The input-output properties of the dentate gyrus have been quantified in this way using both in vivo and in vitro preparations, and we are now focusing on determining the relative contributions of individual subsystems and cell populations of the dentate. The experiments proposed herein will use this nonlinear systems to model the effects of the feedforward and feedback circuitries of the dentate gyrus on granule cell activity and to define the roles of mossy cells and aspiny interneurons in producing the feedback effects.